Power switching apparatus

ABSTRACT

A power switching apparatus includes a plurality of semiconductor switching devices connected in parallel with each other and a plurality of balance resistor units. The plurality of balance resistor units each have one end connected to a control electrode of an associated semiconductor switching device and the other end to which a common control signal is input. Each balance resistor unit is configured to have a resistance value switched between different values depending on whether the plurality of semiconductor switching devices are turned on or turned off in accordance with the control signal.

TECHNICAL FIELD

The present disclosure relates to a power switching apparatus includinga plurality of semiconductor switching devices connected in parallelwith each other and a gate drive circuit for these semiconductorswitching devices. The present invention also relates to a powerswitching apparatus further including a protection circuit for thesesemiconductor switching devices.

BACKGROUND ART

When a plurality of power semiconductor switching devices are connectedin parallel, a closed circuit is formed by respective gate to drain (orgate to source) capacitances of the semiconductor elements and a wiringinductance. In this closed circuit, parasitic oscillation may occurduring turn-on or turn-off of the semiconductor switching devices(parasitic oscillation is likely to occur particularly during turn-offof the semiconductor switching devices). Occurrence of parasiticoscillation may cause breakage of the semiconductor switching devices.This parasitic oscillation is a problem peculiar to the configurationwhere a plurality of semiconductor switching devices are connected inparallel.

In order to avoid this problem, generally a gate resistor having arelatively large resistance value is connected to the gate of eachsemiconductor switching device. For example, according to PatentDocument 1 (Japanese Patent Laying-Open No. 2003-088098), parasiticoscillation is suppressed by a damping resistor on the output terminalside of a gate drive circuit.

CITATION LIST Patent Document

-   PTD 1: Japanese Patent Laying-Open No. 2003-088098

SUMMARY OF INVENTION Technical Problem

Connection of a gate resistor having a relatively large resistance valueas described above leads to a problem that the turn-on time and theturn-off time increase. This is for the reason that the turn-on time andthe turn-off time are determined by the product of the resistance valueof the gate resistor and the gate to source capacitance of thesemiconductor switching device. The increased turn-on time and theincreased turn-off time cause increase of a turn-on loss and increase ofa turn-off loss, respectively. Accordingly, addition of the gateresistor for suppressing parasitic oscillation during turn-off causesundesired increase of not only the turn-off loss but also the turn-onloss.

A similar problem arises when radiation noise due to high switchingspeed of the semiconductor switching device is to be suppressed. Theradiation noise is caused by an abrupt change of the drain voltage andthe drain current. An example is a case where radiation noise occurringwhen a semiconductor switching device is turned on is a problem to besolved. In this case, a gate resistor having a relatively largeresistance value is added to reduce the rate of change of the drainvoltage and the drain current when the semiconductor switching device isturned on, undesired increase of not only the turn-on loss but also theturn-off occurs.

The present disclosure has been made in view of the above problems andis directed to a power switching apparatus including a plurality ofparallel-connected semiconductor switching devices. The presentdisclosure aims to take measures to solve a problem arising in one ofthe turn-on operation and the turn-off operation so as not to increase aloss in the other operation.

Solution to Problem

A power switching apparatus of the present disclosure includes aplurality of semiconductor switching devices connected in parallel witheach other, a plurality of balance resistor units, and a controlcircuit. The plurality of semiconductor switching devices are connectedin parallel with each other and each have a first main electrode, asecond main electrode, and a control electrode. The plurality of balanceresistor units are each associated with a respective one of theplurality of semiconductor switching devices, and each have one endconnected to the control electrode of the associated semiconductorswitching device. The control circuit outputs, to the other end of eachbalance resistor unit, a common control signal for turning on andturning off each semiconductor switching device. Each balance resistorunit is configured to have a resistance value switched between differentvalues depending on whether the plurality of semiconductor switchingdevices are turned on or turned off in accordance with the controlsignal. The balance resistor unit is provided, for plurality of powersemiconductor switching devices connected in parallel, as a balanceresistor for suppressing parasitic oscillation occurring duringswitching of the semiconductor switching devices.

Advantageous Effects of Invention

According to the invention, the resistance value of each balanceresistor unit can be set to different values for turn-on and turn-off ofa plurality of semiconductor switching devices. Therefore, when measuresto solve a problem arising in one of the turn-on and turn-off operationsare taken, increase of a loss in the other operation can be prevented.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram showing a configuration of a power switchingapparatus 100 in a first embodiment.

FIG. 2 is a timing chart showing an operation of power switchingapparatus 100 in FIG. 1.

FIG. 3 is a circuit diagram showing a configuration of a power switchingapparatus 101 in a second embodiment.

FIG. 4 is a circuit diagram showing a configuration of a power switchingapparatus 102 in a third embodiment.

FIG. 5 is a circuit diagram showing a configuration of a power switchingapparatus 103 in a fourth embodiment.

FIG. 6 is a circuit diagram showing a configuration of a combination ofpower switching apparatus 100 in FIG. 1 with a short-circuit protectioncircuit.

FIG. 7 is a timing chart showing an operation of an RTC operationdetermination circuit 30 in FIG. 6.

FIG. 8 is a diagram showing a path for gate current Ig in a normaloperation of power switching apparatus 104 in FIG. 6.

FIG. 9 is a diagram showing a path for gate current Ig in ashort-circuit operation of power switching apparatus 104 in FIG. 6.

FIG. 10 is a circuit diagram showing a configuration of a combination ofpower switching apparatus 102 in FIG. 4 with a short-circuit protectioncircuit.

FIG. 11 is a diagram showing a path for gate current Ig when asemiconductor switching device T2 a in a power switching apparatus 105in FIG. 10 fails due to short circuit.

FIG. 12 is a circuit diagram showing a configuration of a modificationof the combination of power switching apparatus 102 in FIG. 4 with ashort-circuit protection circuit.

FIG. 13 is a diagram showing a path for gate current Ig when only asemiconductor switching device T2 a in a power switching apparatus 106in FIG. 12 fails due to short circuit.

DESCRIPTION OF EMBODIMENTS

In the following, each embodiment is described in detail with referenceto the drawings. The same or corresponding components are denoted by thesame reference characters, and a description thereof is not repeated.

First Embodiment

[Configuration of Power Switching Apparatus 100]

FIG. 1 is a circuit diagram showing a configuration of a power switchingapparatus 100 in a first embodiment. Referring to FIG. 1, powerswitching apparatus 100 includes semiconductor modules Ta, Tb connectedin parallel with each other, and a drive circuit GD.

Semiconductor module Ta includes a power NMOSFET (N-channel Metal OxideSemiconductor Field Effect Transistor) as a semiconductor switchingdevice T1 a connected between a high-voltage side node ND and alow-voltage side node NS, and a diode D1 a. Diode D1 a is connected inantiparallel with semiconductor switching device T1 a (i.e., the drainof the NMOSFET (T1 a) is connected to the cathode of diode D1 a). DiodeD1 a is provided to cause freewheeling current to flow whensemiconductor switching device T1 a is turned off. In FIG. 1, aninternal gate resistor of the NMOSFET (T1 a) is denoted by ra.

Semiconductor module Tb likewise includes a power NMOSFET as asemiconductor switching device T1 b connected between high-voltage sidenode ND and low-voltage side node NS, and a diode D1 b. Diode D1 b isconnected in antiparallel with semiconductor switching device T1 b.Diode D1 b is provided to cause freewheeling current to flow whensemiconductor switching device T1 b is turned off. In FIG. 1, aninternal gate resistor of the NMOSFET (T1 b) is denoted by rb.

Semiconductor switching devices T1 a, T1 b are each aself-arc-extinguishing-type semiconductor device including a first mainelectrode, a second main electrode, and a control electrode, andswitching current flowing between the first and second main electrodeson or off in accordance with a signal applied to the control electrode.FIG. 1 illustrates an example where N-type power MOSFETs are used assemiconductor switching devices T1 a, T1 b. In this case, the first mainelectrode is the source of the NMOSFET, the second main electrode is thedrain of the NMOSFET, and the control electrode is the gate of theNMOSFET.

Drive circuit GD includes balance resistor units Ra, Rb and a controlcircuit 12. Balance resistor unit Ra is connected between an output nodeN1 a and the gate of semiconductor switching device T1 a. Output node N1a branches off from output node N1 of control circuit 12 to output acontrol signal to the control electrode of semiconductor switchingdevice T1 a. Balance resistor unit Rb is connected between an outputnode N1 b and the gate of semiconductor switching device T1 a. Outputnode N1 b branches off from output node N1 of control circuit 12 tooutput a control signal to the control electrode of semiconductorswitching device T1 b. Balance resistor units Ra, Rb are provided asbalance resistors for synchronizing the turn-on/turn off timing ofsemiconductor switching device T1 a with the turn-on/turn off timing ofsemiconductor switching device T1 b. For a plurality ofparallel-connected power semiconductor switching devices, balanceresistor units Ra, Rb are also provided for suppressing parasiticoscillation during turn-on or turn-off of the power semiconductorswitching devices.

More specifically, balance resistor unit Ra includes a diode D2 a and aresistor element R3 a connected in parallel with each other. The cathodeof diode D2 a is connected to the gate of semiconductor switching deviceT1 a, and the anode of diode D2 a is connected to output node N1 a ofcontrol circuit 12. Balance resistor unit Rb likewise includes a diodeD2 b and a resistor element R3 b connected in parallel with each other.The cathode of diode D2 b is connected to the gate of semiconductorswitching device T1 b, and the anode of diode D2 b is connected tooutput node N1 b of control circuit 12.

Control circuit 12 outputs a common control signal for turning on andturning off a plurality of semiconductor switching devices T1 a, T1 b.More specifically, control circuit 12 includes a switch control circuit13, a turn-on NMOSFET 14 as a switching element, a turn-off PMOSFET(P-channel MOSFET) 15 as a switching element, a turn-on gate resistor R1for adjusting the turn-on switching speed, a turn-off gate resistor R2for adjusting the turn-off switching speed, a first DC power supply 10,and a second DC power supply 11. The resistance value of turn-on gateresistor R1 is determined so that a required turn-on switching speed isachieved. The resistance value of turn-off gate resistor R2 isdetermined so that a required turn-off switching speed is achieved. Theturn-on gate resistor may be referred to herein as first resistorelement and the turn-off gate resistor may be referred to herein assecond resistor element.

First and second DC power supplies 10, 11 are connected in series witheach other (the negative node of DC power supply 10 is connected to thepositive node of DC power supply 11). A connection node N3 connectingfirst and second DC power supplies 10, 11 is connected to source N4 a ofthe NMOSFET (T1 a) and source N4 b of the NMOSFET (T1 b). In thefollowing, respective output voltages (power supply voltages) of firstand second DC power supplies 10, 11 are denoted by Vs.

Turn-on gate resistor R1 and NMOSFET 14 are connected in series betweenpositive node N2 of first DC power supply 10 and output node N1 ofcontrol circuit 12. In FIG. 1, while turn-on gate resistor R1 isconnected to the drain of NMOSFET 14, turn-on gate resistor R1 may beconnected to the source of NMOSFET 14. Likewise, turn-off gate resistorR2 and PMOSFET 15 are connected in series between output node N1 ofcontrol circuit 12 and a ground node GND. In FIG. 1, while turn-off gateresistor R2 is connected to the drain of PMOSFET 15, turn-on gateresistor R1 may be connected to the source of PMOSFET 15.

Switch control circuit 13 controls turn-on MOSFET 14 and turn-off MOSFET15 in response to external control signal Sg. In an example in the firstembodiment, switch control circuit 13 turns on MOSFET 14 and turns offMOSFET 15 in response to external control signal Sg at the high level (Hlevel). As a result, semiconductor switching devices T1 a, T1 b areturned on. Switch control circuit 13 turns off MOSFET 14 and turns onMOSFET 15 in response to external control signal Sg at the low level (Llevel). As a result, semiconductor switching devices T1 a, T1 b areturned off.

[Operation of Power Switching Apparatus 100]

An operation of power switching apparatus 100 in FIG. 1 is described.Power semiconductor modules Ta, Tb have the same circuit configurationand balance resistor units Ra, Rb also have the same circuitconfiguration, and therefore, turn-on/turn-off of semiconductorswitching device T1 a occurs almost simultaneously with turn-on/turn-offof semiconductor switching device T1 b. In the following, therefore,chiefly an operation of power semiconductor module Ta is described.

FIG. 2 is a timing chart showing an operation of power switchingapparatus 100 in FIG. 1. FIG. 2 shows, successively from the top,external control signal Sg, gate voltages Vga, Vgb of semiconductorswitching devices T1 a, T1 b, control current (gate current) Ig that isoutput from output node N1 of control circuit 12, drain current Idflowing from high-voltage side node ND to semiconductor switchingdevices T1 a, T1 b, and drain voltage Vd of semiconductor switchingdevices T1 a, T1 b. The horizontal axis represents time (TIME).

Referring to FIGS. 1 and 2, at time t0, external control signal Sgswitches from the L level to the H level. In response to this, turn-onMOSFET 14 of control circuit 12 switches to the ON state and turn-offMOSFET 15 switches to the OFF state. Accordingly, gate current flowsfrom positive node N2 of first DC power supply 10 to semiconductorswitching device T1 a through turn-on gate resistor R1, diode D2 a ofbalance resistor unit Ra, and internal gate resistor ra. As a result, apositive voltage is applied between the gate and the source of firstsemiconductor switching device T1 a. In the case of balance resistorunit Ra in FIG. 1, substantially the whole gate current flows throughforward-direction diode D2 a and no gate current flows through resistorelement R3 a.

At time t20, the gate to source voltage increases to become equal to ormore than a threshold voltage of semiconductor switching device T1 a.Then, semiconductor switching device T1 a becomes conductive (turnedon). Drain current Id flows to semiconductor switching device T1 athrough a main circuit (not shown) connected between the drain and thesource of semiconductor switching device T1 a. The turn-on time at thistime is determined by the product of the total resistance value ofinternal gate resistor ra of semiconductor switching device T1 a andturn-on gate resistor R1, and the gate to source capacitance ofsemiconductor switching device T1 a. The larger the resistance value,the longer the turn-on time.

The period from time t1 to time t2 is a mirror period in which gatevoltage Vg is kept constant by the mirror effect of semiconductorswitching device T1 a. In the mirror period, variation of voltage Vdbetween main electrodes causes variation of the parasitic capacitance ofsemiconductor switching device T1 a, and gate voltage Vg is thereforekept constant. At time t2, the mirror period ends. Then, gate voltage Vgincreases again. At time t3, gate voltage Vga reaches positive powersupply voltage Vs. Accordingly, the turn-on operation ends.

At time t4, external control signal Sg switches from the H level to theL level. In response to this, turn-on MOSFET 14 of control circuit 12switches to the OFF state and turn-off MOSFET 15 switches to the ONstate. Accordingly, gate current flows from the gate of semiconductorswitching device T1 a to ground node GND successively through internalgate resistor ra, resistor element R3 a of balance resistor unit Ra, andturn-off gate resistor R2. As a result, a negative voltage is appliedbetween the gate and the source of first semiconductor switching deviceT1 a. In the case of balance resistor unit Ra in FIG. 1, gate current inthe reverse direction of diode D2 a is blocked, and therefore,substantially the whole gate current flows through resistor element R3a.

As the gate to source voltage decreases to become less than thethreshold voltage of semiconductor switching device T1 a, semiconductorswitching device T1 a is turned off. Then, drain current Id does notflow through the main circuit (not shown) connected between the drainand the source. The turn-off time at this time is determined by theproduct of the total resistance value of internal gate resistor ra ofsemiconductor switching device T1 a, resistor element R3 a of balanceresistor unit Ra, and turn-off gate resistor R2, and the gate to sourcecapacitance of semiconductor switching device T1 a.

At time t5, voltage Vd between main electrodes starts increasing. Then,the period from time t5 to time t6 is a mirror period in which gatevoltage Vg is kept substantially constant. At time t6, the mirror periodends. Then, gate voltage Vg starts decreasing again. At time t7, gatevoltage Vga reaches negative power supply voltage−Vs. Accordingly, theturn-off operation ends.

Effects of First Embodiment

In FIG. 1, if balance resistor units Ra, Rb are not equipped with diodesD2 a, D2 b but equipped with resistor elements R3 a, R3 b only, not onlythe turn-off gate resistance value but also the turn-on gate resistancevalue increases and thus not only the turn-off loss but also the turn-onloss increases. The conventional art often employs such a configurationfor the purpose of suppressing parasitic oscillation during turn-off.

In contrast, in power switching apparatus 100 in the present embodiment,balance resistor unit Ra is formed by parallel-connected resistorelement R3 a and diode D2 a. Cathode of diode D2 a is connected to thegate of semiconductor switching device T1 a. Accordingly, whensemiconductor switching device T1 a is turned on, no gate current Igflows through resistor R3 a. As a result, the value of the turn-on gateresistance is determined by turn-on gate resistor R1 and internal gateresistor ra of power semiconductor module Ta. Even when the resistancevalue of resistor R3 a of balance resistor unit Ra is increased for thepurpose of suppressing parasitic oscillation during turn-off, theturn-on time does not increase. In other words, in power switchingapparatus 100 in the present embodiment, parasitic oscillation occurringduring switching operation can be suppressed without increasing theturn-on loss of parallel-connected semiconductor switching devices T1 a,T1 b.

[Modification]

When increase of the turn-off loss is to be prevented, the polarity ofdiodes D2 a, D2 b forming balance resistor units Ra, Rb is reversed,with respect to the polarity thereof in FIG. 1. Specifically, thecathode of diode D2 a is connected to output node N1 a of controlcircuit 12, and the anode thereof is connected to the gate ofsemiconductor switching device T1 a. The cathode of diode D2 b isconnected to output node N1 b of control circuit 12, and the anodethereof is connected to the gate of semiconductor switching device T1 b.An operation of power switching apparatus 100 in this case is described.In particular, an operation of semiconductor module Ta, balance resistorunit Ra, and control circuit 12 is described.

In response to switching of external control signal Sg from the L levelto the H level, turn-on MOSFET 14 of control circuit 12 switches to theON state and turn-off MOSFET 15 switches to the OFF state. Accordingly,gate current flows from positive node N2 of first DC power supply 10 tosemiconductor switching device T1 a through turn-on gate resistor R1,resistor element R3 a of balance resistor unit Ra, and internal gateresistor ra of power semiconductor module Ta. As a result, a positivevoltage is applied between the gate and the source of firstsemiconductor switching device T1 a.

As the gate to source voltage increases to become equal to or more thanthe threshold voltage of semiconductor switching device T1 a,semiconductor switching device T1 a becomes conductive. The turn-on timeat this time is determined by the product of the total resistance valueof internal gate resistor ra of semiconductor switching device T1 a,resistor element R3 a of balance resistor unit Ra, and turn-on gateresistor R1, and the gate to source capacitance of semiconductorswitching device T1 a.

In contrast, in response to switching of external control signal Sg fromthe H level to the L level, turn-on MOSFET 14 of control circuit 12switches to the OFF state and turn-off MOSFET 15 switches to the ONstate. Accordingly, gate current flows from the gate of semiconductorswitching device T1 a to ground node GND successively through internalgate resistor ra, diode D2 a of balance resistor unit Ra, and turn-offgate resistor R2. As a result, a negative voltage is applied between thegate and the source of first semiconductor switching device T1 a.

As the gate to source voltage decreases to become less than thethreshold voltage of semiconductor switching device T1 a, semiconductorswitching device T1 a is turned off. The turn-off time at this time isdetermined by the product of the total resistance value of internal gateresistor ra of semiconductor switching device T1 a and turn-off gateresistor R2, and the gate to source capacitance of semiconductorswitching device T1 a.

As seen from the above, when semiconductor switching device T1 a isturned off, no gate current flows through resistor element R3 a ofbalance resistor unit Ra. Therefore, even when the resistance value ofresistor element R3 a of balance resistor unit Ra is increased for thepurpose of suppressing parasitic oscillation during switching, theturn-off time does not increase. In other words, in the power switchingapparatus in the modification described above, parasitic oscillationoccurring during switching operation can be suppressed withoutincreasing the turn-off loss of parallel-connected semiconductorswitching devices T1 a, T1 b.

Second Embodiment

[Configuration of Power Switching Apparatus 101]

FIG. 3 is a circuit diagram showing a configuration of a power switchingapparatus 101 in a second embodiment. Power switching apparatus 101 inFIG. 3 differs from power switching apparatus 100 in FIG. 1 in theconfiguration of balance resistor units Ra, Rb. Other features in FIG. 3are similar to those in FIG. 1. Therefore, the same or correspondingcomponents are denoted by the same reference characters, and thedescription thereof is not repeated.

As shown in FIG. 3, balance resistor unit Ra includes a diode D2 a and aresistor element R4 a connected in series with each other and arrangedbetween output node N1 a of control circuit 12 and the gate ofsemiconductor switching device T1 a. Balance resistor unit Ra furtherincludes a resistor element R3 a connected in parallel with thecombination of diode D2 a and resistor element R4 a. The cathode ofdiode D2 a is connected to the gate of semiconductor switching device T1a. The order in which diode D2 a and resistor element R4 a are connectedmay be reversed with respect to the order shown in FIG. 3.

Balance resistor unit Rb likewise includes a diode D2 b and a resistorelement R4 b connected in series with each other and arranged betweenoutput node N1 b of control circuit 12 and the gate of semiconductorswitching device T1 b. Balance resistor unit Rb further includes aresistor element R3 b connected in parallel with the combination ofdiode D2 b and resistor element R4 b. The cathode of diode D2 b isconnected to the gate of semiconductor switching device T1 b. The orderin which diode D2 b and resistor element R4 b are connected may bereversed with respect to the order shown in FIG. 3.

[Operation of Power Switching Apparatus 101]

An operation of power switching apparatus 101 in FIG. 3 is described.Power semiconductor modules Ta, Tb have the same circuit configurationand balance resistor units Ra, Rb also have the same circuitconfiguration, and therefore, turn-on/turn-off of semiconductorswitching device T1 a occurs almost simultaneously with turn-on/turn-offof semiconductor switching device T1 b. In the following, therefore,chiefly an operation of power semiconductor module Ta is described.

In response to switching of external control signal Sg from the L levelto the H level, turn-on MOSFET 14 of control circuit 12 switches to theON state and turn-off MOSFET 15 switches to the OFF state. Accordingly,gate current flows from positive node N2 of first DC power supply 10 tosemiconductor switching device T1 a through turn-on gate resistor R1,resistor elements R3 a, R4 a and diode D2 a of balance resistor unit Ra,and internal gate resistor ra of power semiconductor module Ta. As aresult, a positive voltage is applied between the gate and the source offirst semiconductor switching device T1 a to cause semiconductorswitching device T1 a to be turned on. The turn-on time at this time isdetermined by the product of the total resistance value of internal gateresistor ra of semiconductor switching device T1 a, resistor elements R3a, R4 a of balance resistor unit Ra, and turn-on gate resistor R1, andthe gate to source capacitance of semiconductor switching device T1 a.

In contrast, in response to switching of external control signal Sg fromthe H level to the L level, turn-on MOSFET 14 of control circuit 12switches to the OFF state and turn-off MOSFET 15 switches to the ONstate. Accordingly, gate current flows from the gate of semiconductorswitching device T1 a to ground node GND successively through internalgate resistor ra, resistor element R3 a of balance resistor unit Ra, andturn-off gate resistor R2. As a result, a negative voltage is appliedbetween the gate and the source of first semiconductor switching deviceT1 a to cause semiconductor switching device T1 a to be turned off. Theturn-off time at this time is determined by the product of the totalresistance value of internal gate resistor ra of semiconductor switchingdevice T1 a, resistor element R3 a of balance resistor unit Ra, andturn-off gate resistor R2, and the gate to source capacitance ofsemiconductor switching device T1 a.

Specifically, in the configuration described above, the turn-onresistance value of balance resistor unit Ra is given by:R3a×R4a/(R3a+R4a)  (1)where R3 a, R4 a are resistance values of resistor elements R3 a, R4 a,respectively. The turn-off resistance value of balance resistor unit Rais given by R3 a. It is therefore possible to make the turn-onresistance value of balance resistor unit Ra smaller than the turn-offresistance value of balance resistor unit Ra. As a result, parasiticoscillation during switching can be suppressed, without useless increaseof the turn-on loss of parallel-connected semiconductor switchingdevices. Moreover, in the configuration of the first embodiment shown inFIG. 1, charge passes through only one resistor element between the gateof semiconductor module Ta and the gate of semiconductor module Tbduring parasitic oscillation. In contrast, charge passes throughmultiple resistor elements in the configuration of the secondembodiment, which produces a greater effect of suppressing parasiticoscillation occurring during switching.

[Modification]

When increase of the turn-off loss is to be prevented, the polarity ofdiodes D2 a, D2 b forming balance resistor units Ra, Rb is reversed,with respect to the polarity thereof in FIG. 3. Specifically, thecathode of diode D2 a is connected to output node N1 a of controlcircuit 12, and the cathode of diode D2 b is connected to output node N1b of control circuit 12.

In this case, the turn-on resistance value of balance resistor unit Rais given by R3 a and the turn-off resistance value of balance resistorunit Ra is given by the expression (1) above. It is therefore possibleto make the turn-off resistance value of balance resistor unit Rasmaller than the turn-on resistance value of balance resistor unit Ra.As a result, a resistance value of resistor elements R3 a, R4 a can bedetermined so as to suppress parasitic oscillation during switching,without useless increase of the turn-off loss of parallel-connectedsemiconductor switching devices.

Third Embodiment

[Configuration of Power Switching Apparatus 102]

FIG. 4 is a circuit diagram showing a configuration of a power switchingapparatus 102 in a third embodiment. Power switching apparatus 102 inFIG. 4 differs from power switching apparatus 100 in FIG. 1 in theconfiguration of balance resistor units Ra, Rb. Other features in FIG. 4are similar to those in FIG. 1. Therefore, the same or correspondingcomponents are denoted by the same reference characters, and thedescription thereof is not repeated.

As shown in FIG. 4, balance resistor unit Ra includes a diode D2 a and aresistor element R4 a connected in series with each other and arrangedbetween output node N1 a of control circuit 12 and the gate ofsemiconductor switching device T1 a. Balance resistor unit Ra furtherincludes a resistor element R3 a and a diode D3 a connected in serieswith each other and in parallel with the combination of diode D2 a andresistor element R4 a. The cathode of diode D2 a is connected to thegate of semiconductor switching device T1 a. The cathode of diode D3 ais connected to output node N1 a of control circuit 12. Namely, thepolarity of diode D2 a is opposite to the polarity of diode D3 a. Theorder in which diode D2 a and resistor element R4 a are connected may bereversed with respect to the order shown in FIG. 4, and the order inwhich resistor element R3 a and diode D3 a are connected may also bereversed with respect to the order shown in FIG. 4.

Balance resistor unit Rb likewise includes a diode D2 b and a resistorelement R4 b connected in series with each other and arranged betweenoutput node N1 b of control circuit 12 and the gate of semiconductorswitching device T1 b. Balance resistor unit Rb further includes aresistor element R3 b and a diode D3 b connected in series with eachother and in parallel with the combination of diode D2 b and resistorelement R4 b. The cathode of diode D2 b is connected to the gate ofsemiconductor switching device T1 b. The cathode of diode D3 b isconnected to output node N1 b of control circuit 12. Namely, thepolarity of diode D2 b is opposite to the polarity of diode D3 b. Theorder in which diode D2 b and resistor element R4 b are connected may bereversed with respect to the order shown in FIG. 4, and the order inwhich resistor element R3 b and diode D3 b are connected may also bereversed with respect to the order shown in FIG. 4.

[Operation of Power Switching Apparatus 102]

An operation of power switching apparatus 102 in FIG. 4 is described.Power semiconductor modules Ta, Tb have the same circuit configurationand balance resistor units Ra, Rb also have the same circuitconfiguration, and therefore, turn-on/turn-off of semiconductorswitching device T1 a occurs almost simultaneously with turn-on/turn-offof semiconductor switching device T1 b. In the following, therefore,chiefly an operation of power semiconductor module Ta is described.

In response to switching of external control signal Sg from the L levelto the H level, turn-on MOSFET 14 of control circuit 12 switches to theON state and turn-off MOSFET 15 switches to the OFF state. Accordingly,gate current flows from positive node N2 of first DC power supply 10 tosemiconductor switching device T1 a through turn-on gate resistor R1,resistor element R4 a and diode D2 a of balance resistor unit Ra, andinternal gate resistor ra of power semiconductor module Ta. As a result,a positive voltage is applied between the gate and the source of firstsemiconductor switching device T1 a to cause semiconductor switchingdevice T1 a to be turned on. The turn-on time at this time is determinedby the product of the total resistance value of internal gate resistorra of semiconductor switching device T1 a, resistor element R4 a ofbalance resistor unit Ra, and turn-on gate resistor R1, and the gate tosource capacitance of semiconductor switching device T1 a.

In contrast, in response to switching of external control signal Sg fromthe H level to the L level, turn-on MOSFET 14 of control circuit 12switches to the OFF state and turn-off MOSFET 15 switches to the ONstate. Accordingly, gate current flows from the gate of semiconductorswitching device T1 a to ground node GND successively through internalgate resistor ra, resistor element R3 a and diode D3 a of balanceresistor unit Ra, and turn-off gate resistor R2. As a result, a negativevoltage is applied between the gate and the source of firstsemiconductor switching device T1 a to cause semiconductor switchingdevice T1 a to be turned off. The turn-off time at this time isdetermined by the product of the total resistance value of internal gateresistor ra of semiconductor switching device T1 a, resistor element R3a of balance resistor unit Ra, and turn-off gate resistor R2, and thegate to source capacitance of semiconductor switching device T1 a.

Specifically, in the configuration described above, the turn-onresistance value of balance resistor unit Ra is given by R4 a and theturn-off resistance value of balance resistor unit Ra is given by R3 a,where R3 a and R4 a are respective resistance values of resistorelements R3 a and R4 a, respectively. Thus, the turn-on resistance value(R4 a) of balance resistor unit Ra and the turn-off resistance value (R3a) of balance resistor unit Ra can be adjusted totally independently ofeach other. Therefore, when increase of the turn-on loss is to beprevented, the resistance value of resistor element R3 a forming balanceresistor unit Ra can be set larger to suppress parasitic oscillationduring switching, without influencing the turn-on loss at all. Likewise,when increase of the turn-off loss is to be prevented, the resistancevalue of resistor element R4 a forming balance resistor unit Ra can beset larger to suppress parasitic oscillation during switching operationof semiconductor switching devices connected in parallel, withoutuseless increase of the loss in any one of the turn-on switching and theturn-off switching.

Fourth Embodiment

[Configuration of Power Switching Apparatus 103]

FIG. 5 is a circuit diagram showing a configuration of a power switchingapparatus 103 in a fourth embodiment. Power switching apparatus 103 inFIG. 5 differs from power switching apparatus 100 in FIG. 1 in theconfiguration of control circuit 12 and balance resistor units Ra, Rb.Other features in FIG. 5 are similar to those in FIG. 1. Therefore, thesame or corresponding components are denoted by the same referencecharacters, and the description thereof is not repeated.

As shown in FIG. 5, control circuit 12 includes an output node N10connected to the source of turn-on NMOSFET 14 and an output node N11connected to the source of turn-off PMOSFET 15. An interconnection N10 abranching off from output node N10 connected to the source of turn-onNMOSFET 14 to output a control signal to the control electrode ofsemiconductor switching device T1 a, and an interconnection N10 bbranching off from output node N10 to output a control signal to thecontrol electrode of semiconductor switching device T1 b are arranged.An interconnection N11 a branching off from output node N11 connected tothe source of turn-off PMOSFET 15 to output a control signal to thecontrol electrode of semiconductor switching device T1 a, and aninterconnection N11 b branching off from output node N11 to output acontrol signal to the control electrode of semiconductor switchingdevice T1 b are arranged. Interconnection N10 a and interconnection N11a are connected to the control electrode (gate) of semiconductorswitching device T1 a. Interconnection N10 b and interconnection N11 bare connected to the control electrode (gate) of semiconductor switchingdevice T1 b. Output node N10 connected to the source of turn-on NMOSFET14 may be referred to herein as first output node and output node N11connected to the source of the turn-off PMOSFET may be referred toherein as second output node.

Balance resistor unit Ra includes a resistor element R4 a arrangedbetween output node N10 and the gate of semiconductor switching deviceT1 a (i.e., arranged on interconnection N10 a), and a diode D3 a and aresistor element R3 a connected in series with each other and arrangedbetween output node N11 and the gate of semiconductor switching deviceT1 a (i.e., arranged on interconnection N11 a). The cathode of diode D3a is connected to output node N11. The order in which diode D3 a andresistor element R3 a are connected may be reversed with respect to theorder shown in FIG. 5. Diode D3 a may be connected in series withresistor element R4 a. In this case, the cathode of diode D3 a isconnected to the gate of semiconductor switching device T1 a. In thiscase as well, the order in which diode D3 a and resistor element R4 aare connected is not limited to a specific order.

Likewise, balance resistor unit Rb includes a resistor element R4 barranged between output node N10 and the gate of semiconductor switchingdevice T1 b (i.e., arranged on interconnection N10 b), and a diode D3 band a resistor element R3 b connected in series with each other andarranged between output node N11 and the gate of semiconductor switchingdevice T1 b (i.e., arranged on interconnection N11 b). The cathode ofdiode D3 b is connected to output node N11. The order in which diode D3b and resistor element R3 b are connected may be reversed with respectto the order shown in FIG. 5. Diode D3 b may be connected in series withresistor element R4 b. In this case, the cathode of diode D3 b isconnected to the gate of semiconductor switching device T1 b. In thiscase as well, the order in which diode D3 b and resistor element R4 bare connected is not limited to a specific order.

[Operation of Power Switching Apparatus 103]

An operation of power switching apparatus 103 in FIG. 5 is described.Power semiconductor modules Ta, Tb have the same circuit configurationand balance resistor units Ra, Rb also have the same circuitconfiguration, and therefore, turn-on/turn-off of semiconductorswitching device T1 a occurs almost simultaneously with turn-on/turn-offof semiconductor switching device T1 b. In the following, therefore,chiefly an operation of power semiconductor module Ta is described.

In response to switching of external control signal Sg from the L levelto the H level, turn-on MOSFET 14 of control circuit 12 switches to theON state and turn-off MOSFET 15 switches to the OFF state. Accordingly,gate current flows from positive node N2 of first DC power supply 10 tosemiconductor switching device T1 a through turn-on gate resistor R1,output node 10 a, resistor element R4 a of balance resistor unit Ra, andinternal gate resistor ra of power semiconductor module Ta. As a result,a positive voltage is applied between the gate and the source of firstsemiconductor switching device T1 a to cause semiconductor switchingdevice T1 a to be turned on. The turn-on time at this time is determinedby the product of the total resistance value of internal gate resistorra of semiconductor switching device T1 a, resistor element R4 a ofbalance resistor unit Ra, and turn-on gate resistor R1, and the gate tosource capacitance of semiconductor switching device T1 a.

In contrast, in response to switching of external control signal Sg fromthe H level to the L level, turn-on MOSFET 14 of control circuit 12switches to the OFF state and turn-off MOSFET 15 switches to the ONstate. Accordingly, gate current flows from the gate of semiconductorswitching device T1 a to ground node GND successively through internalgate resistor ra, resistor element R3 a of balance resistor unit Ra,output node N11, and turn-off gate resistor R2. As a result, a negativevoltage is applied between the gate and the source of firstsemiconductor switching device T1 a to cause semiconductor switchingdevice T1 a to be turned off. The turn-off time at this time isdetermined by the product of the total resistance value of internal gateresistor ra of semiconductor switching device T1 a, resistor element R3a of balance resistor unit Ra, and turn-off gate resistor R2, and thegate to source capacitance of semiconductor switching device T1 a.

The above-described configuration provides similar effects to those ofthe third embodiment, and further enables reduction of the number ofcomponents of balance resistor units Ra, Rb relative to the thirdembodiment. The configuration of balance resistor units Ra, Rb may bereplaced with any of respective configurations in the first, second, andthird embodiments described in connection with FIGS. 1, 3, and 4.

Fifth Embodiment

[Overall Configuration of Power Switching Apparatus]

FIG. 6 is a circuit diagram showing a configuration of a combination ofpower switching apparatus 100 in FIG. 1 with a short-circuit protectioncircuit. Semiconductor module Ta in FIG. 6 differs from semiconductormodule Ta in FIG. 1 in that the former additionally includes an RTC(Real-Time Current Control) circuit 20 a. Semiconductor module Tb inFIG. 6 differs from semiconductor module Tb in FIG. 1 in that the formeradditionally includes an RTC circuit 20 b. In other words, RTC circuit20 (20 a, 20 b) is provided for each of semiconductor switching devicesT2 a, T2 b. In semiconductor module Ta in FIG. 6, a semiconductorswitching device T2 a having a sense terminal to is used. Insemiconductor module Tb in FIG. 6, a semiconductor switching device T2 bhaving a sense terminal tb is used.

Drive circuit GD in FIG. 6 differs from drive circuit GD in FIG. 3 inthat the former additionally includes an RTC operation determinationcircuit 30 connected to turn-on gate resistor R1. RTC circuit 20 may bereferred to herein as first short-circuit protection circuit and RTCoperation determination circuit 30 may be referred to herein as secondshort-circuit protection circuit.

Other features in FIG. 6 are the same as those in FIG. 1. Therefore, thesame or corresponding components are denoted by the same referencecharacters, and the description thereof is not repeated.

[Configuration and Operation of RTC Circuit]

RTC circuits 20 a, 20 b each lower the gate to source voltage ofassociated semiconductor switching device T2 a, T2 b when the draincurrent (main circuit current) of associated semiconductor switchingdevice T2 a, T2 b becomes overcurrent, to thereby reduce the draincurrent. In this way, semiconductor switching devices T2 a, T2 b areprotected. RTC circuits 20 a, 20 b have the same circuit configurationand RTC circuit 20 a is therefore described in the following.

As shown in FIG. 6, RTC circuit 20 a includes a sense resistor R5 a, adiode D4 a, a resistor element R6 a, and an NPN bipolar transistor Q1 a.Sense resistor R5 a is connected between sense terminal ta and a node N4a connected to the source of semiconductor switching device T2 a. Theorder in which diode D4 a and resistor element R6 a are connected may bereversed. The base of bipolar transistor Q1 a is connected to senseterminal ta of semiconductor switching device T2 a.

In RTC circuit 20 a having the above-described configuration, when sensecurrent flows through sense terminal ta of semiconductor switchingdevice T2 a, a voltage is generated at sense resistor R5 a (i.e., thesense current is detected by sense resistor R5 a). When the voltagegenerated at sense resistor R5 a exceeds a threshold value, NPNtransistor Q1 a is turned on. As a result, the gate to source voltage ofsemiconductor switching device T2 a decreases, and therefore, the draincurrent (main circuit current) of semiconductor switching device T2 a isreduced.

RTC circuit 20 a in FIG. 6 is merely an example. RTC circuit 20 a maymore generally be configured in other ways, as long as RTC circuit 20 ais configured to include a current detector (R5 a) detecting draincurrent (main circuit current) flowing through the semiconductorswitching device, and a determination processor (Q1 a) reducing the gatevoltage of the semiconductor switching device when the detected draincurrent is higher than a threshold value.

[Configuration and Operation of RTC Operation Determination Circuit]

RTC operation determination circuit 30 determines whether one (at leastone) of RTC circuits 20 a, 20 b is operating or not. When RTC operationdetermination circuit 30 determines that one of RTC circuits 20 a, 20 bis operating, RTC operation determination circuit 30 forcefully blocksthe output of control circuit 12 (forces control circuit 12 to output acontrol signal that causes semiconductor switching device T2 a, T2 b tobe switched to the OFF state). Specifically, RTC operation determinationcircuit 30 includes a delay circuit 31 (mask circuit), a voltagereduction circuit 32, and a PNP bipolar transistor Q2.

Delay circuit 31 is connected in parallel with turn-on gate resistor R1and includes a capacitor C1 and a resistor element R7 connected inseries with each other. One end of resistor element R7 is connected to anode N5 connected to a low-voltage side of turn-on gate resistor R1.

Voltage reduction circuit 32 includes a Zener diode ZD1 and resistorelements R8, R9. The anode of Zener diode ZD1 is connected to the otherend N6 of resistor element R7. Resistor elements R8 and R9 connected inthis order are connected between the cathode of Zener diode ZD1 andpositive node N2 of DC power supply 10.

The emitter of PNP bipolar transistor Q2 is connected to positive nodeN2 of DC power supply 10, and the base of transistor Q2 is connected toa connecting node connecting resistor elements R8 and R9. From thecollector of transistor Q2, a signal representing a result ofdetermination on operation of RTC circuits 20 a, 20 b is output toswitch control circuit 13.

FIG. 7 is a timing chart showing an operation of RTC operationdetermination circuit 30 in FIG. 6. FIG. 7 shows, successively from thetop, external control signal Sg, gate voltages Vga, Vgb of semiconductorswitching devices T1 a, T1 b, control current (gate current) Ig that isoutput from output node N1 of control circuit 12, drain current Id ofsemiconductor switching devices T1 a, T1 b, and drain voltage Vd ofsemiconductor switching devices T1 a, T1 b. FIG. 7 further shows voltageVrg generated at turn-on gate resistor R1 and base to emitter voltageVgf of transistor Q2. In the following, a description is given of anoperation of semiconductor switching device T2 a and balance resistorunit Ra when short-circuit current flows. The same is applied as well tosemiconductor switching device T2 b and balance resistor unit Rb.

Referring to FIGS. 6 and 7, at time t10, external control signal Sgswitches from the L level to the H level. In response to this, turn-onMOSFET 14 of control circuit 12 switches to the ON state and turn-offMOSFET 15 switches to the OFF state. Accordingly, gate current flowsfrom positive node N2 of first DC power supply 10 to semiconductorswitching device T2 a through turn-on gate resistor R1, diode D2 a ofbalance resistor unit Ra, and internal gate resistor ra. As a result, apositive voltage is applied between the gate and the source of firstsemiconductor switching device T2 a. At time t21, semiconductorswitching device T2 a is turned on.

When short circuit occurs, the load is low and therefore, large draincurrent Id (main circuit current) flows relative to the drain current innormal operation. At time t11, the voltage generated at resistor elementR5 a exceeds a threshold voltage, and therefore, transistor Q1 aswitches to the ON state (RTC circuit 20 a switches to the operatingstate). As a result, gate voltage Vga decreases. Further, as RTC circuit20 a switches to the operating state, gate current Ig keeps flowingstill after time t11. While gate current Ig is flowing, capacitor C1 iskept charged, and therefore, the absolute value of base to emittervoltage Vgf of transistor Q2 keeps increasing.

At time t13, gate to emitter voltage Vgf exceeds threshold voltage Vgfonof transistor Q2. Then, transistor Q2 switches to the ON state.Accordingly, a signal that is output from RTC operation determinationcircuit 30 to switch control circuit 13 and represents a result ofdetermination is activated (rises to the H level). As a result, switchcontrol circuit 13 causes gate voltage Vga to become 0 V at time t14.Further, as the result of determination by RTC operation determinationcircuit 30 is output to the external circuit, external control signal Sgswitches from the H level to the L level at time t15.

An additional description is given below of effects of voltage reductioncircuit 32. Threshold voltage Vgfon of transistor Q2 is approximately0.6 V to 1 V. A problem arising from this is as follows. In order forthe absolute value of gate voltage Vgf of transistor Q2 not to exceedthe absolute value of threshold voltage Vgfon at the turn-on time (fromtime t0 to time t3 in FIG. 2) in normal operation, the time constant ofdelay circuit 31 must be set to a relatively large value.

In the circuit shown in FIG. 6 including voltage reduction circuit 32,gate voltage Vgf of transistor Q2 at the time of turn-on is equal to avoltage determined by subtracting a Zener voltage of Zener diode ZD1from a voltage of capacitor C1 and dividing the determined difference byresistor elements R8, R9. In other words, the absolute value of gatevoltage Vgf of transistor Q2 is smaller than the one when no voltagereduction circuit 32 is provided. As a result, the time constant ofdelay circuit 31 can be set to a relatively small value, and therefore,short-circuit protective operation can be made faster.

Voltage reduction circuit 32 is not necessarily a requisite component.Specifically, RTC operation determination circuit 30 at least includesdelay circuit (mask circuit) 31 that outputs a voltage determined bydelaying a change of a terminal to terminal voltage of turn-on gateresistor R1, and a determination circuit (Q2) determining that the RTCcircuit is operating when the output voltage of delay circuit 31 exceedsa threshold value.

[Operation of Power Switching Apparatus 104]

An operation of the power switching apparatus is described, including anoperation of the short-circuit protection circuit.

FIG. 8 is a diagram showing a path for gate current Ig in a normaloperation of power switching apparatus 104 in FIG. 6. FIG. 9 is adiagram showing a path for gate current Ig in a short-circuit operationof power switching apparatus 104 in FIG. 6. In FIGS. 8 and 9, the pathsfor gate current Ig are indicated by bold lines.

Referring to FIG. 8, the normal operation where no short circuit occursis described. In the normal operation, NPN transistors Q1 a, Q1 b are inthe OFF state, and therefore, RTC circuits 20 a, 20 b do not operate. Atthe time of turn-on in the normal operation, gate current Ig flows asshown in FIG. 8 only during a period in which input capacitors ofsemiconductor switching devices T2 a, T2 b are charged, and a voltage isgenerated across turn-on gate resistor R1. In order to prevent PNPtransistor Q2 from being turned on at this time, RTC operationdetermination circuit 30 includes delay circuit 31 (mask circuit) formedby capacitor C1 and resistor R7. Delay circuit 31 delays rising of thevoltage generated across resistor R9, and therefore, transistor Q2 iskept in the OFF state.

Referring to FIG. 9, the short-circuit operation is described. When themain circuit is short-circuited due to malfunction for example of switchcontrol circuit 13 to cause main circuit current of semiconductor moduleTa and main circuit current of semiconductor module Tb to becomeovercurrent simultaneously, current flowing from each of sense terminalsta, tb of semiconductor switching devices T2 a, T2 b also increases inproportion to the main circuit current. As a result, voltages generatedat sense resistors Rya, R5 b increase to thereby cause the base toemitter voltages of NPN transistors Q1 a, Q1 b to increase. When thebase to emitter voltage exceeds a threshold voltage of each of NPNtransistors Q1 a, Q1 b, NPN transistors Q1 a, Q1 b are turned on.

As a result, as shown in FIG. 9, gate current Ig flows successivelythrough turn-on gate resistor R1, diode D2 a of balance resistor unitRa, and diode D4 a and resistor element R6 a in RTC circuit 20 a.Further, gate current Ig flows successively through turn-on gateresistor R1, diode D2 b of balance resistor unit Rb, and diode D4 b andresistor R6 b in RTC circuit 20 b. Further, as NPN transistors Q1 a, Q1b are turned on, the gate to source voltages of semiconductor switchingdevices T2 a, T2 b decrease, and accordingly main circuit current Id isreduced.

At this time, the gate to source voltage of semiconductor switchingdevice T2 a is equal to a voltage generated at resistor element R6 a.The voltage at resistor element R6 a is a voltage determined by dividingpower supply voltage Vs by a resistance value of turn-on gate resistorR1 and a half of a resistance value of resistor element R6 a. Likewise,the gate to source voltage of semiconductor switching device T2 b isequal to a voltage generated at resistor element R6 b. The voltage atresistor element R6 b is a voltage determined by dividing power supplyvoltage Vs by a resistance value of turn-on gate resistor R1 and a halfof a resistance value of resistor element R6 b. The resistance value ofresistor element R6 a and the resistance value of resistor element R6 bdescribed above are equal to each other. The resistance values ofbalance resistor units Ra, Rb are negligible because they are equal tothe resistance values at the time of turn-on in the normal operation,i.e., the on resistances of diodes D2 a, D2 b.

After the operation of the RTC circuit, gate current Ig still keepsflowing, and therefore, a voltage is kept generated at turn-on gateresistor R1. The voltage generated at turn-on gate resistor R1 is avoltage determined by dividing power supply voltage Vs by a resistancevalue of turn-on gate resistor R1 and a half of a resistance value ofresistor element R6 a. As a result, when a voltage generated at resistorelement R9 exceeds an operational threshold voltage of PNP transistorQ2, PNP transistor Q2 is turned on. As a result, switch control circuit13 forcefully blocks external control signal Sg.

The voltage at resistor element R9 has a value depending on the voltageat turn-on gate resistor R1. Therefore, the voltage divider ratiodetermined by turn-on gate resistor R1 and resistor R6 a of RTC circuit20 a influences the operational accuracy of RTC operation determinationcircuit 30. In the case for example of the configuration of theconventional art in which diodes D2 a, D2 b are not provided in balanceresistor units Ra, Rb, increase of the resistance values of balanceresistor units Ra, Rb with the purpose of suppressing parasiticoscillation during turn-off results in decrease of the voltage generatedacross turn-on gate resistor R1. Due to this, operation of RTC operationdetermination circuit 30 is slowed. In the worst case, the RTC operationdetermination circuit may not operate when short circuit occurs. Incontrast, in the configuration of the present embodiment, even when theresistance values of balance resistor units Ra, Rb (i.e., resistancevalues of resistor elements R3 a, R3 b) at the time of turn-off areincreased with the purpose of suppressing parasitic oscillation, thevalue of turn-on gate resistor R1 is not influenced. As a result, thevoltage generated across turn-on gate resistor R1 after operation of RTCcircuits 20 a, 20 b is kept constant regardless of the values ofresistor elements R3 a, R3 b of the balance resistor units. Accurateoperation of RTC operation determination circuit 30 is thereforepossible.

As seen from the above, power switching apparatus 104 in the presentembodiment provides similar effects to those in the first embodiment,and enables RTC operation determination circuit 30 to operate correctlywhen short circuit occurs.

Sixth Embodiment

[Configuration of Power Switching Apparatus 105]

FIG. 10 is a circuit diagram showing a configuration of a combination ofpower switching apparatus 102 in FIG. 4 with a short-circuit protectioncircuit. Semiconductor modules Ta, Tb in FIG. 10 differ fromsemiconductor module Ta in FIG. 4 in that the former modules Ta, Tbfurther include respective RTC circuits 20 a, 20 b. The exampleconfiguration of RTC circuits 20 a, 20 b is the same as the onedescribed in connection with FIG. 6. The description thereof istherefore not repeated.

In addition, in semiconductor module Ta in FIG. 10, a semiconductorswitching device T2 a having a sense terminal to is used and, insemiconductor module Tb, a semiconductor switching device T2 b having asense terminal tb is used.

Drive circuit GD in FIG. 10 differs from drive circuit GD in FIG. 4 inthat the former further includes an RTC operation determination circuit30 a connected to resistor element R4 a of balance resistor unit Ra andan RTC operation determination circuit 30 b connected to resistorelement R4 b of balance resistor unit Rb. The configuration of RTCoperation determination circuits 30 a, 30 b is the same as that of RTCoperation determination circuit 30 described in connection with FIG. 6.Therefore, these RTC operation determination circuits are denoted by thesame reference character as RTC operation determination circuit 30 inFIG. 6 except for suffixes “a” and “b” and the description thereof isnot repeated. Suffixes “a” and “b” indicate that the associatedcomponents correspond to RTC operation determination circuits 30 a, 30b, respectively. RTC operation determination circuits 30 a, 30 b may beconnected to the opposite ends of associated balance resistor units Ra,Rb.

Other features in FIG. 10 are the same as those in FIG. 4. Therefore,the same or corresponding components are denoted by the same referencecharacters, and the description thereof is not repeated.

[Operation of Power Switching Apparatus 105]

A description is given of a short-circuit protection operation whensemiconductor switching device T2 a, which is one of parallel-connectedsemiconductor switching devices T2 a, T2 b, is short-circuited due to acertain failure.

FIG. 11 is a diagram showing a path for gate current Ig whensemiconductor switching device T2 a in power switching apparatus 105 inFIG. 10 fails due to short circuit. In FIG. 11, the path for gatecurrent Ig is indicated by a bold line.

When semiconductor switching device T2 a is short-circuited due to acertain failure, sense current flowing from sense terminal to ofsemiconductor switching device T2 a increases in proportion to maincurrent between main electrodes. Then, when a voltage generated at senseresistor Rya, i.e., the base to emitter voltage of NPN transistor Q1 aexceeds a threshold voltage, NPN transistor Q1 a is turned on. As aresult, as shown in FIG. 11, gate current Ig flows successively throughturn-on gate resistor R1, resistor element R4 a and diode D2 a ofbalance resistor unit Ra, and diode D4 a and resistor element R6 a inRTC circuit 20 a. As NPN transistor Q1 a is turned on, the gate tosource voltage of semiconductor switching device T2 a decreases, andaccordingly main current Id is reduced.

At this time, the gate to source voltage of semiconductor switchingdevice T2 a is equal to the voltage generated at resistor element R6 a.The voltage at resistor element R6 a is a voltage determined by dividingpower supply voltage Vs by the resistance value of turn-on gate resistorR1, the resistance value of resistor element R4 a of balance resistorunit Ra, and the resistance value of resistor element R6 a.

After the operation of RTC circuit 20 a, gate current Ig still keepsflowing, and therefore, the voltage is kept generated across resistorelement R4 a of balance resistor unit Ra. The voltage across resistorelement R4 a of balance resistor unit Ra is a voltage determined bydividing power supply voltage Vs by a resistance value of turn-on gateresistor R1, a resistance value of resistor element R4 a of balanceresistor unit Ra, and a resistance value of resistor element R6 a. As aresult, when a voltage across resistor element R9 a exceeds anoperational threshold voltage of PNP transistor Q2 a, PNP transistor Q2a is turned on. As a result, switch control circuit 13 forcefully blocksexternal control signal Sg.

The voltage across resistor element R9 a has a value depending on thevoltage at resistor element R4 a of balance resistor unit Ra. Therefore,the voltage divider ratio determined by turn-on gate resistor R1,resistor element R4 a of balance resistor unit Ra, and resistor R6 a ofRTC circuit 20 a influences the operational accuracy of RTC operationdetermination circuit 30 a. In the case of the present embodiment, theresistance value of balance resistor unit Ra at the time of turn-on andin the short-circuit operation is determined by the resistance value ofresistor element R4 a, and the resistance value of balance resistor unitRa at the time of turn-off is determined by the resistance value ofresistor element R3 a. In other words, the resistance value of balanceresistor unit Ra at the time of turn-on is not influenced by theresistance value of balance resistor unit Ra at the time of turn-off. Itis therefore possible to reduce the resistance value of turn-on gateresistor R1 and increase the resistance value of resistor element R4 aof balance resistor unit Ra. As a result, the voltage at resistor R4 aof balance resistor unit Ra after the operation of RTC circuit 20 a canbe set relatively large to cause RTC operation determination circuit 30a to operate accurately.

The circuit configuration of balance resistor unit Ra is the same as thecircuit configuration of balance resistor unit Rb, and the circuitconfiguration of semiconductor module Ta is the same as the circuitconfiguration of semiconductor module Tb. Therefore, when semiconductorswitching device T2 b fails due to short circuit, short-circuitprotection can be performed speedily and accurately in a similar mannerto the above-described one. Further, the resistance values of resistorelements R4 a, R4 b of balance resistor units Ra, Rb can be increased toachieve the effect of suppressing parasitic oscillation during switchingthat occurs when semiconductor switching devices are connected inparallel.

It is supposed that, in the fifth embodiment (power switching apparatus104 in FIG. 6), only semiconductor switching device T2 a isshort-circuited due to a certain failure and only RTC circuit 20 aoperates. In this case, the voltage across turn-on gate resistor R1 is avoltage determined by dividing power supply voltage Vs by the resistancevalue of turn-on gate resistor R1 and the resistance value of resistorelement R6 a. Therefore, the operational accuracy of RTC operationdetermination circuit 30 is lowered relative to the case whereshort-circuit current flows through semiconductor switching devices T2a, T2 b simultaneously. In contrast, the present embodiment sets theresistance value of turn-on gate resistor R1 to 0Ω. Accordingly, in boththe case where one of semiconductor switching devices T2 a, T2 b isshort-circuited and the case where both are short-circuitedsimultaneously, the voltage of resistor element R4 a after the operationof the RTC circuit is equal to the voltage determined by dividing powersupply voltage Vs by the resistance value of resistor element R4 a ofbalance resistor unit Ra and the resistance value of resistor element R6b. It is therefore possible to cause RTC operation determination circuit30 a to operate correctly with the same accuracy in both the cases.

Effects of Sixth Embodiment

As seen from the above, when short-circuit current flows through atleast one of semiconductor switching devices T2 a, T2 b in powerswitching apparatus 105 in the present embodiment, it is possible tocause RTC operation determination circuits 30 a, 30 b to operatecorrectly. As a result, fast and correct short-circuit protection isachieved.

In the fifth embodiment, as the number of parallel-connectedsemiconductor switching devices T2 a, T2 b, . . . is larger, the lowerthe operational accuracy of the RTC operation determination circuit whenany one of the semiconductor switching devices is short-circuited due toa certain failure. In contrast, in the present embodiment, even when thenumber of parallel-connected semiconductor switching devices T2 a, T2 b,is larger, the operational accuracy of RTC operation determinationcircuits 30 a, 30 b, remains the same. The present embodiment istherefore effective particularly when the number of parallel-connectedsemiconductor switching devices is large.

RTC operation determination circuits 30 a, 30 b in the presentembodiment can each be connected to the opposite ends of associatedbalance resistor unit Ra, Rb or associated resistor element R4 a, R4 bdescribed in connection with FIG. 3. RTC operation determinationcircuits 30 a, 30 b in the present embodiment can each be connected alsoto the opposite ends of associated balance resistor unit Ra, Rb orassociated resistor element R4 a, R4 b described in connection with FIG.5.

Seventh Embodiment

[Configuration of Power Switching Apparatus 106]

FIG. 12 is a circuit diagram showing a configuration of a modificationof the combination of power switching apparatus 102 in FIG. 4 with ashort-circuit protection circuit. Semiconductor modules Ta, Tb in FIG.12 differ from semiconductor module Ta in FIG. 4 in that the formersemiconductor modules Ta, Tb further include RTC circuits 20 a, 20 b,respectively. An example configuration of RTC circuits 20 a, 20 b is thesame as the one described in connection with FIG. 6. Therefore, thedescription thereof is not repeated.

Further, for semiconductor module Ta in FIG. 12, a semiconductorswitching device T2 a having a sense terminal to is used and, forsemiconductor module Tb in FIG. 12, a semiconductor switching device T2b having a sense terminal tb is used.

Drive circuit GD in FIG. 12 further includes diodes D5 a, D5 b. Thecathode of diode D5 a is connected to a connecting line connectingbalance resistor unit Ra to the gate of semiconductor switching deviceT2 a. The cathode of diode D5 b is connected to a connecting lineconnecting balance resistor unit Rb to the gate of semiconductorswitching device T2 b. The anode of diode D5 a and the anode of diode D5b are connected to a common connection node N9.

RTC operation determination circuit 30 is connected between output nodeN1 of control circuit 12 and connection node N9. The configuration ofRTC operation determination circuit 30 is the same as the one describedin connection with FIG. 6. Specifically, RTC operation determinationcircuit 30 includes delay circuit 31 (mask circuit), voltage reductioncircuit 32, and PNP bipolar transistor Q2. Delay circuit 31 includescapacitor C1 and resistor element R7 connected in series between outputnode N1 and connection node N9 (capacitor C1 is connected so that thedistance from capacitor C1 to output node N1 is shorter than thedistance from resistor element R7 to output node N1).

Other features in FIG. 12 are the same as those in FIG. 4. Therefore,the same or corresponding components are denoted by the same referencecharacters, and the description thereof is not repeated. Theconfiguration of balance resistor units Ra, Rb and the configuration ofcontrol circuit 12 in FIG. 12 may be any of modifications of FIG. 1 andconfigurations in FIGS. 3 and 5. When the present embodiment is combinedwith the configuration in FIG. 5, RTC operation determination circuit 30is connected between output node N10 of control circuit 12 andconnection node N9. Connection node N9 may be referred to herein asfirst connection node.

[Operation of Power Switching Apparatus 106]

A description is given of a short-circuit protection operation whensemiconductor switching device T2 a, which is one of parallel-connectedsemiconductor switching devices T2 a, T2 b, is short-circuited due to acertain failure.

FIG. 13 is a diagram showing a path for gate current Ig whensemiconductor switching device T2 a in power switching apparatus 106 inFIG. 12 fails due to short circuit. In FIG. 13, the path for gatecurrent Ig is indicated by a bold line.

When semiconductor switching device T2 a is short-circuited due to acertain failure, sense current flowing from sense terminal to ofsemiconductor switching device T2 a increases in proportion to maincurrent between main electrodes. Then, when the voltage across senseresistor Rya, i.e., the base to emitter voltage of NPN transistor Q1 aexceeds a threshold voltage, NPN transistor Q1 a is turned on. As aresult, as shown in FIG. 13, gate current Ig flows successively throughturn-on gate resistor R1, resistor element R4 a of balance resistor unitRa, and diode D4 a and resistor element R6 a in RTC circuit 20 a. As NPNtransistor Q1 a is turned on, the gate to source voltage ofsemiconductor switching device T2 a decreases, and accordingly maincurrent Id is reduced.

At this time, the gate to source voltage of semiconductor switchingdevice T2 a is equal to the voltage generated at resistor element R6 a.The voltage at resistor element R6 a is a voltage determined by dividingpower supply voltage Vs by the resistance value of turn-on gate resistorR1, the resistance value of resistor element R4 a of balance resistorunit Ra, and the resistance value of resistor element R6 a.

After the operation of RTC circuit 20 a, gate current Ig still keepsflowing, and therefore, the voltage is kept generated across turn-ongate resistor R1. The voltage across turn-on gate resistor R1 is avoltage determined by dividing power supply voltage Vs by a resistancevalue of turn-on gate resistor R1, a resistance value of resistorelement R4 a of balance resistor unit Ra, and a resistance value ofresistor element R6 a. As a result, when a voltage generated at resistorelement R9 exceeds an operational threshold voltage of PNP transistorQ2, PNP transistor Q2 is turned on. As a result, switch control circuit13 forcefully blocks external control signal Sg.

The voltage across resistor element R9 has a value depending on thevoltage at resistor element R4 a of balance resistor unit Ra. Therefore,the voltage divider ratio determined by turn-on gate resistor R1,resistor element R4 a of balance resistor unit Ra, and resistor R6 a ofRTC circuit 20 a influences the operational accuracy of RTC operationdetermination circuit 30. In the case of the present embodiment, theresistance value of balance resistor unit Ra at the time of turn-on andin the short-circuit operation is determined by the resistance value ofresistor element R4 a, and the resistance value of balance resistor unitRa at the time of turn-off is determined by the resistance value ofresistor element R3 a. In other words, the resistance value of balanceresistor unit Ra at the time of turn-on is not influenced by theresistance value of balance resistor unit Ra at the time of turn-off. Itis therefore possible to increase the resistance value of resistorelement R4 a of balance resistor unit Ra and decrease the resistancevalue of turn-on gate resistor R1. As a result, the voltage at resistorR4 a of balance resistor unit Ra after the operation of RTC circuit 20 acan be set relatively large to cause RTC operation determination circuit30 to operate accurately.

The circuit configuration of balance resistor unit Ra is the same as thecircuit configuration of balance resistor unit Rb, and the circuitconfiguration of semiconductor module Ta is the same as the circuitconfiguration of semiconductor module Tb. Therefore, when semiconductorswitching device T2 b fails due to short circuit, short-circuitprotection can be performed speedily and accurately in a similar mannerto the above-described one. Further, the resistance values of resistorelements R4 a, R4 b of balance resistor units Ra, Rb can be increased tosuppress parasitic oscillation during switching that occurs whensemiconductor switching devices are connected in parallel.

It is supposed that, in the fifth embodiment (power switching apparatus104 in FIG. 6), only semiconductor switching device T2 a isshort-circuited due to a certain failure and only RTC circuit 20 aoperates. In this case, the voltage across turn-on gate resistor R1 is avoltage determined by dividing power supply voltage Vs by the resistancevalue of turn-on gate resistor R1 and the resistance value of resistorelement R6 a. Therefore, the operational accuracy of RTC operationdetermination circuit 30 is lowered relative to the case whereshort-circuit current flows through semiconductor switching devices T2a, T2 b simultaneously. In contrast, the present embodiment sets theresistance value of turn-on gate resistor R1 to 0Ω. Accordingly, in boththe case where one of semiconductor switching devices T2 a, T2 b isshort-circuited and the case where both are short-circuitedsimultaneously, the voltage of resistor element R4 a after the operationof the RTC circuit is equal to the voltage determined by dividing powersupply voltage Vs by the resistance value of resistor element R4 a ofbalance resistor unit Ra and the resistance value of resistor element R6b. It is therefore possible to cause RTC operation determination circuit30 a to operate correctly with the same accuracy in both the cases.

Effects of Seventh Embodiment

Power switching apparatus 106 in the present embodiment provides similareffects to those in the sixth embodiment. While the sixth embodimentrequires the same number of RTC operation determination circuits as thenumber of parallel-connected semiconductor switching devices, thepresent embodiment requires one RTC operation determination circuitregardless of the number of parallel-connected semiconductor switchingdevices, and therefore can suppress increase of the cost due to increaseof the number of components and/or increase of the control circuit area.

<Modifications Common to the Embodiments>

Instead of turn-on MOSFET 14 and turn-off MOSFET 15 forming gate drivecircuit GD, bipolar transistors may be used. MOSFETs employed assemiconductor switching devices T1 a, T1 b forming semiconductor modulesTa, Tb may be replaced with IGBTs (Insulated Gate Bipolar Transistors).While two semiconductor switching devices T1 a, T1 b are connected inparallel, three or more semiconductor switching devices may be connectedin parallel.

The material for semiconductor switching devices T1 a, T1 b is notlimited to Si (silicon), but may be wide-bandgap semiconductor such asSiC (silicon carbide), GaN (gallium nitride), and C (diamond). Thewide-bandgap semiconductor switching device is suitable for fastswitching. In the case where no diode is provided in balance resistorunits Ra, Rb like the conventional art, not only the turn-off resistancevalue but also the turn-on resistance value increases and therefore notonly turn-off loss but also turn-on loss increases. In such a case, thefast switching devices of wide-bandgap semiconductor elements are noteffectively used. In contrast, balance resistor units Ra, Rb may beconfigured as shown in FIGS. 1, 3, 4, and 5 to suppress parasiticoscillation occurring during switching operation, without increasing theloss involved in any one of turn-on switching operation and turn-offswitching operation. Therefore, the expensive wide-bandgap semiconductorelements are not broken. The characteristics of the wide-bandgapsemiconductor element suitable for fast switching can thus be utilizedeffectively.

<Other Applications>

The power switching apparatus in each embodiment may also be applied tosuppress radiation noise due to large voltage variation dV/dt andcurrent variation dl/dt between the drain and the source ofsemiconductor switching devices T1 a, T1 b. Specifically, when radiationnoise occurring upon turn-off is a problem to be solved, any of theconfigurations shown in FIGS. 1, 3, 4, and 5 can be used as aconfiguration of balance resistor units Ra, Rb to limit radiation noiseoccurring upon turn-off without increasing the turn-on loss. On thecontrary, when radiation noise occurring upon turn-on is a problem to besolved, the configuration in FIGS. 1 and 3 where the polarity of thediodes is opposite can be employed as a configuration of balanceresistor units Ra, Rb, or an appropriate resistance value of resistorelement R4 a in FIG. 4 may be determined or an appropriate value ofresistor element R4 a in FIG. 5 may be determined, to thereby limitradiation noise upon turn-on without increasing the turn-off loss.

It should be construed that embodiments disclosed herein are given byway of illustration in all respects, not by way of limitation. It isintended that the scope of the present invention is defined by claims,not by the description above, and encompasses all modifications andvariations equivalent in meaning and scope to the claims.

REFERENCE SIGNS LIST

10 first DC power supply; 11 second DC power supply; 12 control circuit;13 switch control circuit; 20, 20 a, 20 b RTC circuit; 30, 30 a, 30 bRTC operation determination circuit; 31 delay circuit; 32 voltagereduction circuit; 100-104 power switching apparatus; GD drive circuit;Id drain current (main current); Ig gate current; N1 output node; N2positive node; N3 connection node; ND high-voltage side node; NSlow-voltage side node; Ra, Rb balance resistor unit; Sg external controlsignal; T1 a, T1 b, T2 a, T2 b semiconductor switching device; Ta, Tbsemiconductor module

The invention claimed is:
 1. A power switching apparatus comprising: a plurality of semiconductor switching devices connected in parallel with each other, the semiconductor switching devices each including a first main electrode, a second main electrode and a control electrode; a control circuit including: at least one output node to output a control signal for turning on and turning off each of the semiconductor switching devices; a first resistor element to adjust a turn-on switching speed of each of the semiconductor switching devices; and a second resistor element to adjust a turn-off switching speed of each of the semiconductor switching devices; a plurality of balance resistor units each associated with a respective one of the plurality of semiconductor switching devices and connected between the control electrode of the associated semiconductor switching device and the at least one output node, the balance resistor units each being configured to suppress parasitic oscillation between the semiconductor switching devices, the parasitic oscillation occurring during at least one of turn-on and turn-off of the semiconductor switching devices, the balance resistor units each being configured to have a resistance value switched between different values depending on whether the semiconductor switching devices are turned on or turned off in accordance with the control signal; a plurality of first protection circuits each: associated with a respective one of the plurality of semiconductor switching devices; and reducing a voltage between the control electrode and the first main electrode of the associated semiconductor switching device when detecting overcurrent flowing between the first main electrode and the second main electrode of the associated semiconductor switching device; and a second protection circuit configured to: detect current flowing through an interconnection provided for supplying the control signal; determine, based on the detected current, whether at least one of the plurality of first protection circuits is in operating state; and when an associated protection circuit of the plurality of first protection circuits is in operating state, change the control signal for causing each of the semiconductor switching devices to be turned off.
 2. The power switching apparatus according to claim 1, wherein the control circuit includes a first output node as the at least one output node, the balance resistor units each include: a first rectifying element connected between the first output node of the control circuit and the control electrode of the associated semiconductor switching device; and a third resistor element connected in parallel with the first rectifying element, the first rectifying element has an anode connected directly to the first output node of the control circuit, the control circuit includes: a first switching element connected between a power supply node and the first output node of the control circuit; and a second switching element connected between a ground node and the first output node, the first resistor element is connected in series with the first switching element between the power supply node and the first output node, when the first switching element is in ON state and the second switching element is in OFF state, the control circuit outputs the control signal from the first output node for switching each of the semiconductor switching devices to ON state, and the second protection circuit determines, based on a voltage generated at the first resistor element, whether at least one of the plurality of first protection circuits is in operating state.
 3. The power switching apparatus according to claim 1, wherein the control circuit includes a first output node as the at least one output node, the balance resistor units each include: a first rectifying element and a third resistor element connected in series with each other between the first output node of the control circuit and the control electrode of the associated semiconductor switching device; and a fourth resistor element connected in parallel with a combination of the first rectifying element and the third resistor element, the first rectifying element is configured to block current in a direction from the control electrode to the first output node, the second protection circuit is provided for each of the balance resistor units, and the second protection circuit determines whether an associated protection circuit of the plurality of first protection circuits is in operating state, based on a voltage generated at the third resistor element of the associated balance resistor unit.
 4. The power switching apparatus according to claim 1, wherein the control circuit includes a first output node as the at least one output node, the balance resistor units each include: a first rectifying element and a third resistor element connected in series with each other between the first output node of the control circuit and the control electrode of the associated semiconductor switching device; and a second rectifying element and a fourth resistor element connected in series with each other and in parallel with a combination of the first rectifying element and the third resistor element, the first rectifying element is configured to block current in a direction from the control electrode to the first output node, the second rectifying element is configured to block current in a direction from the first output node to the control electrode, the second protection circuit is provided for each of the balance resistor units, and the second protection circuit determines, based on a voltage generated at the third resistor element of the associated balance resistor unit, whether an associated protection circuit of the plurality of first protection circuits is in operating state.
 5. The power switching apparatus according to claim 1, wherein the control circuit includes, as the at least one output node, a first output node connected with a power supply node and a second output node connected with a ground node, the balance resistor units are each connected between the control electrode of the associated semiconductor switching device and each of the first output node and the second output node, the balance resistor units each include: a third resistor element connected between the first output node of the control circuit and the control electrode of the associated semiconductor switching device; a fourth resistor element connected between the second output node of the control circuit and the control electrode of the associated semiconductor switching device; and a first rectifying element connected in series with one of the third resistor element and the fourth resistor element, the second protection circuit is provided for each of the balance resistor units, and the second protection circuit determines, based on one of a voltage generated at the third resistor element of the associated balance resistor unit and a voltage across the associated balance resistor unit, whether an associated protection circuit of the plurality of first protection circuits is in operating state.
 6. The power switching apparatus according to claim 1, wherein the control circuit includes a first output node as the at least one output node, the balance resistor units each include: a first rectifying element and a third resistor element connected in series with each other between the first output node of the control circuit and the control electrode of the associated semiconductor switching device; and a second rectifying element and a fourth resistor element connected in series with each other and in parallel with a combination of the first rectifying element and the third resistor element, the first rectifying element blocks current in a direction from the control electrode to the first output node, the second rectifying element blocks current in a direction from the first output node to the control electrode, the power switching apparatus further comprises a plurality of third rectifying elements each including a cathode connected to a connecting line between an associated one of the balance resistor units and the control electrode of an associated one of the semiconductor switching devices, the third rectifying elements each have an anode connected to a common first connection node, the second protection circuit is connected between the first output node and the first connection node, and the second protection circuit determines, based on a voltage generated at each of the balance resistor units, whether at least one of the plurality of first protection circuits is in operating state.
 7. The power switching apparatus according to claim 1, wherein the control circuit includes, as the at least one output node, a first output node connected with a power supply node and a second output node connected with a ground node, the balance resistor units are each connected between the control electrode of the associated semiconductor switching device and each of the first output node and the second output node, the balance resistor units each include: a third resistor element connected between the first output node of the control circuit and the control electrode of the associated semiconductor switching device; a fourth resistor element connected between the second output node of the control circuit and the control electrode of the associated semiconductor switching device; and a first rectifying element connected in series with one of the third resistor element and the fourth resistor element, the power switching apparatus further comprises a plurality of third rectifying elements each including a cathode connected to a connecting line between an associated one of the balance resistor units and the control electrode of an associated one of the semiconductor switching devices, the third rectifying elements each including an anode connected to a common first connection node, the second protection circuit is connected between the first connection node and one of the first output node and the second output node, and the second protection circuit determines, based on a voltage generated at each of the balance resistor units, whether at least one of the plurality of first protection circuits is in operating state.
 8. The power switching apparatus according to claim 1, wherein the semiconductor switching devices are each a self-arc-extinguishing-type semiconductor device made from a wide-bandgap semiconductor wider in bandgap than silicon.
 9. The power switching apparatus according to claim 8, wherein the wide-bandgap semiconductor is any one of silicon carbide, gallium nitride, and diamond. 